Use atomics for double-checked locks (SingletonPtr + __cxa_guard)

[kjbracey]

Description

Make SingletonPtr safe using atomics

SingletonPtr's implementation wasn't totally safe - see "C++ and the Perils of Double-Checked Locking" by Meyers and Alexandrescu. No problems observed in practice, but it was potentially susceptible to compiler optimisation (or maybe even SMP issues).

Now that we have atomic loads and stores, the function can be made safe, avoiding any potential races for threads that don't take the lock: ensure that the unlocked load is atomic, and that the pointer store is
atomic.

See https://preshing.com/20130930/double-checked-locking-is-fixed-in-cpp11/ for more discussion.

Use atomics in __cxa_guard functions

Similar to SingletonPtr, use atomic accesses when loading the guard word outside the lock, and when storing, to ensure no races for threads that don't take the lock.

Lack of atomics unlikely to be a problem in current builds, but code could conceivably be subject to reordering if link-time optimisation was enabled.

Pull request type

[ ] Fix
[X] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

@pan-, @c1728p9

[pan-]

The changes looks good and now static initialisation follows C++11 requirements.

Is there any argument still in favour of using SingletonPtr over static initialisation ?

Foo& getFoo {
    static Foo foo;
    return foo;
} 

SingletonPtr<Foo> foo;

[kjbracey]

Is there any argument still in favour of using SingletonPtr over static initialisation ?

Not much, apart from its syntactic handiness. The get is transparent inside the operator -> or *.

If there was a way to achieve the syntactic handiness using a C++ static thing inside the template, that would be ideal.

Only other distinction is SingletonPtr never invokes destructors, but that's already been nobbled in the C++ library for statics by yourself, as I recall.

[cmonr]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 12 of 12 test jobs passed
Build number : 1
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